The invention relates to a developing system for use in a copying process such as an electrophotographic copying process, and more particularly to such a system for developing an electrostatic latent image on a member capable of retaining an electric charge.
In a copying process such as an electrophotographic copying process, an electrostatic latent image is formed on a member such as a copying sheet which is capable of retaining an electric charge, and this latent image is subsequently developed. For example, in the electrophotographic copying process, the prior practice has been to charge a photosensitive material, which is subsequently exposed to an image of an article to be copied to have an electrostatic latent image thereof formed thereon. The latent image is then developed and transferred onto a transfer sheet, followed by the fixing thereof. The developing step comprises supplying a developer to the surface of the photosensitive material to develop the electrostatic latent image.
As one of the developing techniques of this kind, a self-bias developing technique has been proposed in which a developer is supplied onto a member on which an electrostatic latent image is formed and which is opposed to an electrically floating, conductive plate (hereinafter referred to as developing electrode). In the self-bias developing technique, the developing electrode is maintained electrically floating so that a potential is induced on the developing electrode by the charge on the image portion of the member in order to attract the toner on the non-image portion of the member to the developing electrode, thereby preventing the background smearing and improving the contrast of the image. However, where the contrast in the image of an original itself is low or where the area of the image portion is small as compared to that of the non-image portion, a sufficiently high potential may not be induced on the developing electrode from the image portion of the member to achieve the self-bias effects, that is, preventing the background smearing and improving the contrast of the image. In addition, the potential of the developing electrode varies depending upon the potential and the size of the image portion, and this results in a disadvantage that the toner on an image area of low density may be attracted toward the developing electrode to cause the loss of such image area when the potential of the developing electrode is too high. While the application of a constant bias voltage from a d.c. source to the developing electrode has been proposed, too high a potential of the developing electrode causes a disadvantage of removing an image area of low density, while too low a potential of the developing electrode can not prevent the background smearing and improve the contrast in the image.
The present inventors have previously proposed a developing system for the electrophotographic copying process in which the potential of the developing electrode is induced by the potential of a part of a photosensitive material which is maintained free from image formation and which is maintained at a constant potential when charged during the charging step. However, such system still suffered from the disadvantages that the induced potential may happen to be too high and that where a plurality of developing electroes are provided, the potential may vary from electrode to electrode, thus precluding a uniform image characteristic over the entire surface of the photosensitive material.